Simple. You set the pleaseDoNotPirateThisAppPrettyPlease
flag to 1 in your plist.
I'm sorry if I sound offensive but noob developers asking for an easy way to protect their apps from piracy threads shit me something chronic.
If you are a noob developer, concentrate on your app not sucking first, then worry about piracy. Your energy will be far better spent on releasing a polished app rather than worrying about a handful of people that run cracked versions.
FFS, iPhone is hands down the best platform for getting paid for your apps. Not many people run Cydia, worrying about those is simply ridiculous.
This is not the answer you are looking for and I you could perceive that I called you a noob who writes sucky apps, whatever, but it is the right way to go about it. Concentrate on improving experience for people that are paying and forget about those who will never pay.